A nuclear reactor normally includes a pressure vessel containing the reactor core and confining a coolant, such as a pressurized water coolant. The pressure vessel's wall has a generally cylindrical shape and, as part of a protective system, the pressure vessel is enclosed by a concrete wall which functions as both protection in the event of a rupture of the presssure vessel and to provide a biological shield. This protective concrete wall is desirably radially spaced from the pressure vessel's wall to provide room for insertion between the two of a suitable device for monitoring the condition of the pressure vessel's wall from time to time. This space desirably should not be obstructed so as to prevent such monitoring which may require the insertion of an ultrasonic, pressure vessel wall, monitoring device between the two walls.
At the same time, it is desirable to protect the biological shield's concrete wall surrounding the pressure vessel's wall, from thermal shock, and also mechanical shock in the unfortunate event of the rupture of the reactor pressure vessel's wall.
Thus it can be seen that there is the dual problem of accessibility to the reactor pressure vessel's wall within the space between it and the cylindrical wall of the concrete biological shield, while at the same time there is the need for having this space occupied by an element or elements protecting the concrete biological shield's wall both from thermal radiation and possibly mechanical shock in the event the pressure vessel's wall ruptures.